In order to improve theoretical thermal efficiency of a spark ignition gasoline engine, increasing the geometrical compression ratio of the engine is effective. For example, Patent Document 1 shows a high compression spark-ignition direct-injection engine with a geometrical compression ratio of 14 or higher. An engine with a high compression ratio is subject to knocking, when the operation range is in a low-speed, and high-load range including a full load range. Patent Document 1 also teaches adjusting the time of closing an intake valve to reduce the effective compression ratio in the low-speed, high-load range. The ignition time is advanced by reducing the knocking as much as possible, thereby increasing the engine torque.
For example, as shown in Patent Document 2, the combustion type of compressing and igniting a lean air-fuel mixture is known as a technique improving both of the exhaust emission and the thermal efficiency. In an engine performing the compression ignition combustion, increasing the geometrical compression ratio improves both of the compression end pressure and the compression end temperature, and is thus advantageous in stabilizing the compression ignition combustion. On the other hand, in a low load operation range, although the compression ignition combustion is possible, preignition combustion occurs, which rapidly raises the pressure (dP/dt) with an increase in the engine load. Due to noise vibration harshness (NVH) constraints, the range, in which the compression ignition combustion is performed, is difficult to expand to the high load side. As shown in Patent Document 2, even the engine performing the compression ignition combustion do not usually perform the compression ignition combustion but spark ignition combustion by driving a spark plug in a higher load operation range.
Patent Document 3 shows an engine switching between compression ignition combustion and spark ignition combustion in accordance with the operating mode of the engine, and teaches introducing EGR gas into a cylinder in transition from the compression ignition combustion to the spark ignition combustion and enriching the air-fuel ratio as compared to the stoichiometric air-fuel ratio, thereby reducing knocking.